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Astronomers Measure How Fast A Supermassive Black Hole Spins

For the first time, astronomers have been able to determine how fast a supermassive black hole spins – which is fast enough to push the limits of the laws of physics.

Astronomers using observations from NASA’s NuStar space telescope and the ESA’s XMM-Newton have finally solved an astronomical mystery. By doing so, they’ve finally been able to find a definitive measurement for how fast a supermassive black hole spins.

“This is hugely important to the field of black hole science,” said NuStar scientist Lou Kaluzienski in a NASA press release.

The main problem with determining how fast that a black hole can spin is that there were two different models that both did a pretty good job of explaining what astronomers see when they look at black holes. However, those models give very different rates of spin for the black hole.

Astronomers typically use X-ray telescopes when they examine black holes. The X-rays emanating from the black holes will be at different frequencies depending on what type of material that the X-rays are bouncing off of. For example, hydrogen will look different than iron. Those results show up on a graph as “peaks” at different points, and astronomers in particular look at the iron peak to see how sharp it is. If it’s not sharp, then something is happening to the iron.

Astronomers had two models to explain those iron results, shown in the figure below. In the first model, the gravitational distortions caused by the black hole itself was causing the iron to spread out. If that was the case, then astronomers could use the data from the iron peak to derive the spin of the black hole. In the second model, the iron peak is simply being obscured by the hot gasses that surround the black hole. If the second model was accurate, then astronomers can’t use the iron to determine spin.

With the new study, the astronomers were able to rule out the second model entirely. That means that the iron is being distorted by the gravity of the black hole, and so astronomers were able to determine its spin. That was thanks to NuStar’s ability to better observe high-energy X-rays.

“If I could have added one instrument to XMM-Newton, it would have been a telescope like NuSTAR,” said XMM-Newton Project Scientist Norbert Schartel in the NASA release. “The high-energy X-rays provided an essential missing puzzle piece for solving this problem.”

With that issue settled, the astronomers then took the next step of ascertaining the speed of the black hole they were observing, which is about 56 million light years away in the Galaxy NGC 1365. The black hole itself is about 2 million times as massive as our own Sun.

And what was that speed? Well – that’s hard to put your finger on. It’s about 84% of the speed theoretically allowed by Einstein’s theory of relativity. You could think of that as being close to the speed of light, but it’s difficult to translate into a particular velocity. That’s because both the speed and intense gravity of the Black Hole create conditions that make a mathematical conversion to, say, miles per hour extremely difficult. But however it converts, the fact is that its moving incredibly fast.

These findings will make this and other supermassive black holes even more interesting subjects of study for scientists going forward. That’s both because it allows them to see what happens to objects at the extremes of Relativity, and because it provides still more information about the evolution of the universe.

“These monsters, with masses from millions to billions of times that of the sun, are formed as small seeds in the early universe and grow by swallowing stars and gas in their host galaxies, merging with other giant black holes when galaxies collide, or both,” said the study’s lead author, Guido Risaliti in the NASA release.

“The black hole’s spin is a memory, a record, of the past history of the galaxy as a whole,” he added in a separate statement.

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